1. Field of Use
The present invention relates to digital apparatus for recovering binary information recorded on magnetic media, such as disk, diskettes, tapes.
2. Prior Art
Among the methods of recording binary information on magnetic media, the most frequently used are frequency modulation (FM) also known as single density and modified frequency modulation (MFM) or double density. Successively read out binary 1 or 0 bits recorded in FM are identified by the presence or absence, respectively, of a pulse in the center of contiguous recorded cells. Each cell is defined by two timing pulses. The first one defines the beginning of the cell and the second one defines the beginning of the subsequent cell. The time interval of a cell varies according to the media used. For instance, in the case of an 8-inch diskette and FM recording, the cell has a nominal length of 4 microseconds. Therefore, the nominal time intervals between two subsequent pulses may be 2 microseconds or 4 microseconds. In the case of 51/4 inch diskette, the nominal length of a cell is generally 8 microseconds.
In the case of MFM recording, successively read out binary 1 or 0 bits are also identified by the presence or absence, respectively, of a pulse in the center of contiguous cells. However, MFM recording differs from FM recording in that the timing pulse, defining the beginning of a cell, is absent when a pulse representative of a binary 1 information is present in the center of such cell or in the preceding one. For MFM also, the time interval of the cell depends on the recording media. For instance, in case of an 8-inch diskette and MFM recording, the nominal length of the cell is 2 microseconds. Therefore, the nominal interval time between two subsequent pulses can be 2, 3 or 4 microseconds. In the case of a 51/4 inch diskette, the nominal length of a cell is generally 4 microseconds. Further information on FM or MFM recording methods can be found in the IBM document GA 21-9257-1 entitled, "IBM Two Side Diskette Original Equipment Manufacturers Information--Second Edition" dated Nov. 1977.
The pulse sequence read out from the magnetic media support is applied to an input of a recovery system which supplies to an output, the binary information related to the input pulse sequence. Such pulse sequence periodically includes a so-called synchronization field (generally of 6 or 12 bytes), containing a predetermined number of pulses corresponding to a plurality of contiguous cells in which all "1" information bits or all "0" information bits have been recorded. The synchronization field is used by the recovery system for locking in and for establishing if a pulse detected in the input is a timing pulse or a pulse representative of a recorded information bit. The recovery system is therefore able to correctly detect information recorded on the magnetic medium on the basis of the time interval between two subsequent pulses and the nature of such pulses.
Unfortunately, data recovery through only measurement of the time interval between subsequent pulses is not reliable, since such interval may present a substantial deviation from its nominal value resulting in the misinterpretation of the pulse sequence during the recovery phase. Such deviation results from two main causes. The first is due to speed changes in the magnetic media, that is, in the rotational speed tolerances of the motor which drives the magnetic media. The second is due to the so-called phenomenon of peak-shift of the recorded pulse. As it is well known in the art, such shift is primarily due to the mutual influence of adjacent pulses. As known, such shift can be considered zero only when the recorded pulse density is constant, that is, if the interval between subsequent pulses is always equal. Clearly, such situation does not exist in FM and MFM recording except for the synchronization field. Therefore, the recovery system needs apparatus to correct for the causes of such errors.
The most known of these apparatuses are those which make use of an analog phase lock oscillator. Digital phase lock circuits have been recently suggested. Among the phase lock circuits, the apparatus disclosed by European patent application No. 84107390.1 of June 22, 1984, published on May 15, 1985 with No. 0141028 which corresponds to U.S. Pat. No. 4,672,482, issued on June 9, 1987 can be considered as exemplary of the state of the art. According to such patent application, the problem of recovering digital information recorded on magnetic media is solved by providing circuits able to identify, through the measurement of the actual interval between subsequent pulses, the synchronization field which is not affected by peak-shift but only by a possible speed error, therefore establishing the speed error of the magnetic media relative to a nominal speed during a time interval comprising a suitable number of read out pulses. This information which is updated at each synchronization field detection, is used during the reading out of subsequent read out pulses to correct the interval measured between subsequent read out pulses, providing a measurement interval affected only by the peak-shift error of pulses n and n-1 defining the measured interval. This information is applied to a peak-shift recovery unit together with coded information (supplied in feedback by the same recovery unit) defining the amount and the direction of the peak-shift of read pulse n-1.
The deduction of the peak-shift of read pulse n-1 from the correct measured duration allows the calculation of the nominal duration of the interval N between read pulses n-1 and n and the amount and the direction of the peak-shift of pulse n. The peak-shift of pulse n is sent back to discriminate between the nominal duration of the subsequent interval N+1 and the peak-shift of the subsequent read pulse n+1 and so on. In practice, to avoid the propagation and the accumulation of measurement errors, the peak-shift recovery system supplies in feedback not the peak-shift measurement obtained by a difference between measurements, but an "equivalent" code representative of nominal and actual duration of a determined number of intervals between immediately preceding read pulses.
The approach disclosed by the mentioned patent application is extremely efficacious and provides a high discriminating capability, better than that offered by a number of analog circuits and by other digital circuits. It substantially follows a correction concept in two phases: a speed correction based on a precise speed measurement, speed being detected when peak-shift phenomena are missing, and a subsequent peak-shift recovery. However, this approach is inherently affected by a limitation. That is, it cannot be used when the variations in media speed occur with a frequency equal or higher than the speed measurement frequency. It has been further verified that in case of low cost disks or tape unit, particularly "disk drivers" for flexible diskettes with 51/4 inch diameters, speed swinging can occur with frequency of about 1 KHz and an amplitude of .+-.5+8% relative to the nominal speed. When speed swing of this frequency occurs, the operation of the discriminating apparatus disclosed by the mentioned application is seriously jeopardized.
It would be desirable to have a discriminating apparatus able to substantially offer the same performance as the apparatus disclosed by the mentioned application but also be able to operate with magnetic media subject to swings of up to 1 KHz. This is required to enable the generalized use of low cost actuating units.